1. Field of the Invention
The present invention relates to a broadcast message in a broadband wireless communication system. More particularly, the present invention relates to a method and apparatus for synchronizing a broadcast message to minimize interference caused by beamforming.
2. Description of the Related Art
A mobile communication system has limited system performance and capacity due to a wireless channel property such as a Doppler effect, multi-path fading, and co-channel interference between cells or within the cells. A beamforming technique is one technique capable of increasing overall system capacity for solving issues of performance degradation. An example of a mobile communication system that implements the beamforming technique is a system based on the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard, which is hereafter referred to as an IEEE 802.16 system.
In the beamforming technique, a Base Station (BS) receives feedback of information regarding each wireless channel from each Mobile Station (MS) and forms a beam optimized for each MS. There are various types of beamforming techniques, and one of the most widely used beamforming techniques is zero-forcing beamforming.
FIG. 1A illustrates an example of beamforming in a conventional broadband wireless communication system.
Referring to FIG. 1A, each of BSs 100, 110, and 120 forms a beam. FIG. 1A illustrates an ideal case where all BSs simultaneously perform beamforming transmission without interfering with a neighbor BS. That is, each of the BSs 100, 110, and 120 multiplies downlink traffic to be transmitted to each MS with a beamforming coefficient so that a beam is formed in a direction of each MS. In the ideal case, beamforming is achieved by a zero-forcing algorithm while avoiding collision of beams in each of the BSs 100, 110, and 120.
While data messages may be transmitted from one BS to each MS, there may be broadcast messages (i.e., system information, etc.) that are simultaneously transmitted to all MSs located in a coverage of a specific BS. The data message to be transmitted to each MS may be transmitted by forming a beam according to a beamforming technique. However, the beamforming technique may not be used for a broadcast message which has to be received by all MSs. That is, the broadcast message has to be transmitted simultaneously in all directions as in the conventional method. When a broadcast message to which the beamforming technique is not applied and a data message transmitted using beamforming are simultaneously generated, severe interference may occur between a beamformed data message of a neighbor BS and the broadcast message to which beamforming is not applied.
FIG. 1B illustrates an example of beamforming and broadcasting in a conventional broadband wireless communication system.
Referring to FIG. 1B, each of BSs 150 and 160 are transmitting a data message transmitted using beamforming and BS 170 is transmitting a broadcast message in its cell coverage area 175 without using beamforming. Here, BSs 150 and 160 form beams by applying a beamforming coefficient to data to be transmitted to corresponding MSs, and BS 170 transmits a broadcast message to be received by all MSs in its cell coverage area 175. In this case, the BS 150 forms a beam in a direction different from a direction of the BS 170, and thus does not interfere with the BS 170. On the other hand, the BS 160 forms a beam in the same direction as the BS 170, and thus interferes with the BS 170.
As described above, a wireless communication system, such as an IEEE 802.16 system, has a problem due to the occurrence of interference resulting from broadcast messages being independently generated and transmitted in respective BSs. In addition, the occurrence of interference may result in a decrease of a cell throughput by 20˜30%.
Therefore, a need exists for an apparatus and method to address interference caused by beamforming.